Sonja Jejčič^1,2, Lucia Kleint^3,4, & Petr Heinzel²
1. Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia
2. Astronomical Institute, The Czech Academy of Sciences, Ondřejov, Czech Republic
3. University of Applied Sciences and Arts Northwestern Switzerland, Windish, Switzerland
4. Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

The density distribution in flare loops and mechanisms of the continuum emission are subjects of current investigations^[1,2]. On 2017 September 10, a very bright loop system appeared near the west solar limb during the gradual phase of a large X8.2 class flare, which was one of the strongest flares during the solar cycle 24. It was well visible in all passbands of SDO/AIA and in the white-light (WL) “continuum” channel of SDO/HMI at the wavelength 6173Å of the FeI line. A growing flare loop system was visible for about one hour in WL images as shown in Figure 1.

Figure 1| Temporal evolution of SDO/HMI WL loops and their intensities. The left panel shows the HMI WL difference images with a cut through the flare loop (dotted red line). The solid line in the right panel shows calibrated intensities with the pre-flare subtracted in CGS units along the marked red dotted line. The dashed line shows the pre-flare intensity divided by 10.

We analyzed the HMI WL loop brightness by considering four relevant emission processes, i.e., the Thomson scattering (Th) on loop electrons, hydrogen Paschen (Pa) and Brackett (Br) recombination continua, and hydrogen free–free (ff) continuum emission for optically thin flare loop system, assuming pure hydrogen plasma. The contribution of individual emission processes to the WL emission radiation (I_WL) as function of temperature (T) is shown in Figure 2 for three representative electron densities (n_e). The total WL radiation intensity becomes a quadratic function of the electron density, for a given wavelength, temperature and effective loop thickness (D_eff). Therefore, absolutely calibrated SDO/HMI intensities can be converted into the electron densities at each pixel, for a given temperature and effective thickness. Because SDO data do not provide temperature and effective thickness, we computed the electron density for a grid of typical loop temperatures between 6000 K and 10⁶ K and for representative effective thicknesses between 200 km and 20,000 km. We then converted the observed WL brightness map into an electron-density map for two representative temperatures 10⁴ K and 5×10⁵ K and two effective thicknesses 5000 km and 20,000 km as shown in Figure 3.

Figure 2| Contribution of individual processes (Thomson, free-bound, free-free) to the flare loop WL emission as a function of temperature at 10⁴ km above the solar surface for effective thickness 1000 km and for n_e = 10¹¹ cm^-3 (left panel), n_e = 10¹² cm^-3 (middle panel), and n_e = 10¹³ cm^-3 (right panel). The Thomson continuum only dominates for low electron densities. At high densities, the Paschen and Brackett continua dominate at lower temperatures, and the free-free emission at high temperatures. Therefore, the flare loop WL emission can be due to both cool as well as hot loop structures.

Figure 3| An example of the electron density map for four selected models for a snapshot at 16:11:03.8 UT of the WL loop (see second panel in Figure 1). Units in x and y directions are in arcsec. It is well visible that the electron density at the top of loops is highest and then decreases toward the limb in all cases.

The densities we obtain are between 10¹² and 10¹³ cm^-3, and mainly depend on the estimate of the line-of-sight extension of the loop arcade. Such high densities at the loop tops have not yet been obtained from HMI, while they are common just above the limb at chromospheric heights^[2]. Note that such loop densities are also expected on eruptive stars^[3]. The linear polarization of the Thomson continuum should be detectable by HMI and can be used to derive both the electron density and thickness (see Ref. [4]). Note that the Thomson scattering dominates at low electron densities seen in solar prominences, but the radiation intensity is too low to be detected by HMI, therefore, we do not see off-limb prominences by HMI.

For details of this work see Jejčič et al. 2018, ApJ, 867,134.

References

[1] Martínez Oliveros, J.-C., Krucker, S., Hudson, H. S. et al. 2014, ApJ Lett, 780, L28
[2] Heinzel, P., Kleint, L., Kašparová, J., & Krucker, S. 2017, ApJ, 847, 48
[3] Heinzel, P., & Shibata, K. 2018, ApJ, 859, 143
[4] Saint-Hilaire, P., Schou, J., Martínez Oliveros, J.-C., et al. 2014, ApJ Lett, 786, L19